Inks comprising a water-soluble dye such as an acid dye, direct dye or basic dye dissolved in a glycol-based solvent and water are currently widely used as inkjet inks. In order to ensure favorable ink stability, water-soluble dyes that have a high degree of solubility in water are most commonly used. As a result, inkjet recorded items generally exhibit poor water resistance, and if water is spilt on a recorded portion, the dye tends to bleed very easily.
In order to improve the water resistance and weather resistance, solvent-based inks in which a pigment is dispersed within a high boiling point solvent have been developed (for example, Japanese Patent Laid-Open No. 2001-164157 and Japanese Patent Laid-Open No. 2002-302629). However, with these types of inks, the solvent within the ink does not readily volatilize, meaning drying of the ink by evaporation is problematic. Accordingly, printing onto non-absorbent printing substrates such as polyvinyl chloride resin sheets is impossible.
Furthermore, methods that use of a ketone-based solvent such as cyclohexanone or isophorone, or a nitrogen-containing heterocyclic compound such as 2-pyrrolidone or N-methyl-2-pyrrolidone, dissolves the surface of the non-absorbent printing substrate, have recently been proposed thereby improves the fixing properties of the ink (for example, Japanese Patent Laid-Open No. 2005-60716 and Japanese Patent Laid-Open No. 2005-15672). However, because some of these solvents suffer from health and safety problems (they are considered Type 2 Organic Solvents (substances that are clearly hazardous to humans)), they require local ventilation equipment to deal with the problem of strong odors, as defined within the workplace environment assessment standards. Furthermore, because of the strong dissolution properties of these solvents, problems such as dissolution of the pigments and corrosion of the printing head materials may occur, meaning handling is also problematic.
Polyvinyl chloride resins are used almost exclusively as the inkjet printing substrates for outdoor advertising and the like because of their favorable levels of weather resistance, and resistance to chemicals such as alcohol resistance, gasoline resistance and detergent resistance. Vinyl chloride-vinyl acetate resins are used as the binder resins for inkjet inks used for printing onto polyvinyl chloride resin substrates as they exhibit favorable adhesion to the polyvinyl chloride resin substrate along with favorable weather resistance and chemical resistance. For example, inkjet inks prepared by dissolving a vinyl chloride-vinyl acetate resin in a polyoxyethylene glycol dialkyl ether are known (for example, WO2004/007626, pamphlet). However, because polyoxyethylene glycol dialkyl ethers exhibit poor solubility of vinyl chloride-vinyl acetate resins, the thixotropy of the inks is high, meaning the discharge stability is unsatisfactory. Furthermore, if an alcohol-modified vinyl chloride-vinyl acetate resin is used in order to improve the solubility, then the alcohol resistance of the ink coating deteriorates.